The latter, also known as transfer machines, consist essentially of a series of work stations arranged round a worktable which, depending on the type of machine, may have its axis of rotation arranged horizontally or vertically; the table, rotating about its axis with controlled movement, carries the workpieces mounted on it by production personnel at an appropriate loading station, along the various stations at which a programmed cycle of operations is carried out.
The aforesaid stations comprise each at least one work unit and it is to the latter that the present invention refers.
As is already known, these units have a spindle intended to support a tool, which is customarily mounted in such a manner that apart from the rotary movement about its own work axis (generally indicated Z axis) it can also carry out translational movements along said axis.
For this purpose the spindle is commonly housed inside a coaxial sleeve, with respect to which it can rotate freely but to which it is rigidly connected with regard to the axial translational movements mentioned; consequently, when the sleeve which does not have a possibility of rotation, is caused to advance or return along the Z axis with respect to a workpiece to be machined, the spindle also makes similar movements independently of its state of rotation. It will be understood that by so doing, it is possible to carry out screw-threading or other machining of workpieces requiring a helical trajectory to be imparted to the tool.
The axial movements of the spindle and of the sleeve are obtained with hydraulic or electromechanical systems which act externally to the latter, whereas with regard to the rotation of the spindle, motors are used which are coupled to the latter by means of various types of transmission such as, for example, toothed belts and pulleys, reduction mechanisms with gears, and the like.
It is, however, important to take into account the fact that in order to impart to the spindle its rotary work movement while at the same time leaving it free to move axially in translation together with the sleeve, it is necessary to arrange suitable devices at the level of its connection to the corresponding drive motor.
Indeed, in the known operating units the motor is fixedly mounted with respect to the spindle (that is to say, it does not move in translation with it) which according to a technical solution currently widely used, for being driven has on its rearward end longitudinal splines, coupled coaxially with a shell which is also splined inside: on the shell there are then mounted pulleys or other mechanical elements suitable for receiving the motion from the motor, such that the spindle is caused to rotate by the shell owing to the coupling of their respective splines which, at the same time, allow relative movement in an axial direction as desired.
An example of a work unit having this structural configuration is described in Italian Patent Application No. MI 93 A 002548, already published and the owner thereof is the same applicant of the present application.
In the light of what has been explained, it is possible to understand that such a design of an operating unit, although it might have the advantage of being simple and reliable, nevertheless has some drawbacks which limit its performance and therefore, indirectly, also that of the unit itself.
For example, it is necessary to consider that the bulkiness resulting from the presence of the motor with the corresponding mechanical transmission, being it a toothed belt or a gear or other means, is not negligible and may create difficulties, especially in the case of machine tools having a large number of operating units: machine tools having 8 or more work stations are not in fact rare in practice, and in such circumstances the bulkiness referred to above becomes a factor that has a strongly negative effect on the complexity of the machine and therefore on the difficulty of producing the latter.
Furthermore, the inertias coming in to play and which affect the rotation of the spindle, are high in the known work units; this makes the control of such rotation difficult and can therefore reduce the precision of the machining operations carried out by the unit. The higher is the speed of rotation of the spindle, the more marked these negative effects are, and therefore they represent an intrinsic link in the achievement of higher performances and productivity of the machine tools.